Gas Chromatography of Monohydric Phenols via 0-Methylation A. C. Bhattacharyya, Asit Bhattacharjee, 0. K. Guha, and A. N. Basu Central Fuel Research Institute, P. 0. FRI, Dt. Dhanbad, Bihar, India THEOBJECT of the present work was to develop a uniform general procedure for complete 0-methylation of various phenols and to consolidate the quantitative gas-liquid chromatographic study of mixtures of such aryl methyl ethers. The importance of converting such polar compounds like phenols to their ethers before analyzing in a gas chromatograph has been amply demonstrated in some excellent papers (1-4) on silylation of phenols. Conversion of phenols to their ethers (silyl or methyl) leads to an increase in their volatility and decrease in polarity, thus giving rise to peaks without tailing. Blocking of the hydroxyl groups via etherification inhibits the formation of intra- and intermolecular hydrogen bonds; hence interaction with the stationary phase is reduced resulting in symmetrical peaks and shorter elution times. Earlier two papers described almost simultaneously the gas chromatography of methyl ethers of phenols. In the first paper, Carruthers, Johnstone, and Plimmer (5) presented the relative retention volumes of the methyl ethers of phenol, cresols, six dimethylphenols and two dihydroxybenzenes on Apiezon-M grease. However, no experimental procedures for etherification of the phenols were given. The second paper written by G. Bergmann and D. Jentzsch (6) presented a more detailed treatment of the problem. These authors studied etherification of phenol, three methylphenols, six dimethyl phenols, and two ethylphenols. Methylation was carried out by treating the solutions of phenols in aqueous sodium hydroxide with dimethylsulfate (DMS). However, conversion to the methyl ethers was far from quantitative. The yield of the anisoles in case of the dimethyl phenols was 85 i 3 z and as such, a correction factor based on such yields had to be used in quantitative analysis of phenol mixtures. Unless the etherification reaction can be made complete, quantitative gas chromatography of mixtures of phenols via this procedure will be seriously affected by the elution peaks of unreacted phenols. Thus a synthetic mixture of five phenols (phenol, 0-cresol, 2,6-xylenol, 2,4,6- and 2,3,5-trimethylphenols) which are completely resolved on an Apiezon L column, when chromatographed after methylation by the procedure of Bergmann, showed eight distinct peaks and none of the peaks except the first one was due to pure aryl methyl ether. Thus, the need for a more quantitative and suitable method for 0(1) . . S. H. Lamer. P. Pantages. , and I. Wender. Chem. Ind. (London). 1958, 1664.- ' (2) . , D. W. Grant and G. A. Vauehan. "Gas Chromatogravhv 1962," M. Van Swaay, Ed., ButteFworfhs, London, 1962,p i O i . (3) R. W. Freedman and P. P. Croitoru, ANAL.CHEM.,36, 1389 (1964). (4) R. W. Freedman and C. D. Charlier, ibid., p 1880. ( 5 ) W. Carruthers, R. A. W. Johnstone, and J. R. Plimmer, Chem. Ind. (London),1958, 331. ( 6 ) G. Bergmann and D. Jentzsch, 2. Anal. Chern., 164, 10 (1958).
methylation of phenols including highly hindered phenols was felt. The critical step in converting weakly acidic and sterically hindered phenols into their methy ethers consists in prevention of hydrolysis of their sodium salts; hence, use of metallic sodium was preferred to that of aqueous sodium hydroxide to convert the phenols to their salts before reacting with DMS. Some polymethylene phenols have been methylated in this way (7). Thirty-four phenols with different substituent alkyl groups in various positions have been completely methylated by the authors using this procedure. After a general procedure for complete etherification of phenols had been established, the validity of this method for quantitative analysis was checked with two synthetic mixtures. Finally, one tar acid obtained from high temperature carbonization of coal and two tar acids obtained from mild hydrogenation of Assam coal were analysed by GLC via formation of anisoles. EXPERIMENTAL
Apparatus. Analyses were carried out in a Perkin-Elmer 810 Model Gas Chromatograph with a flame ionization detector, fitted with a I-mV Honeywell recorder. Nitrogen was used as the carrier gas at a flow rate of 20 cc per minute. Injection block was kept at 300 "C. Two columns were used for analysis both having Apiezon L as the stationary phase. Column 1 was of 11-foot length, '/*-inch 0.d. copper tubing packed with -80 +IO0 mesh celite coated with 8% Apiezon L. This column was operated at 140 "C. The second column was purchased from Perkin-Elmer (column Qa-) 6-foot stainless steel tube, 0.d. '/*-inch packed with 10% Apiezon L on celite (-80 +IO0 mesh). This was used at 180 "C. In isothermal operations, peak areas obtained by multiplying peak height by the width at half height were used. Reagents. Sodium metal AR. Dioxan Analar BDH,Sodium dried. Dimethyl sulfate BDH-washed successively with ice cold water and cold saturated sodium bicarbonate solution and distilled. The distillate was kept over anhydrous potassium carbonate. Phenols-authentic specimens. Procedure for 0-Methylation of Phenols. Sodium metal was molecularized under toluene and most of the toluene was decanted off. Molecular sodium four times in excess of the theoretical was added to phenol (weight 30 to 500 mg) and dissolved in dry Dioxan (10 ml). The mixture was refluxed for 2 hours. The flask was cooled in a bath of ice water, and pure dry DMS in an amount 4 times theoretical requirement was added slowly through the condenser with shaking. (7) H. Kaemmerar and H. Schweikert, Mukromol. Chem., 83, 188 (1965); CA, 63,4187f(1965). VOL. 40, NO. 12, OCTOBER 1968
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OCH3
1140~Cl
@HI
I
0"":
H3C / CH CH3
(I4OoC) p: c
e J I
1
I
I
I
I
I
9
0
7
6
5
4
I
I
3
2 MlNS
I
I
I
0
-
Figure 2. Isothermal analysis of a synthetic mixture after 0methylation
d
12
-
I
I
0
4
4
0
MlNS
Figure 1. Comparison of extent of methylation by two methods (see text)
Table I. Relative Retention Times of Methyl Ethers of Some Low Boiling Phenols (Apiezon L at 140 "C) Compound Toluene Anisole 2-Methylanisole 4-Methylanisole 3-Methylanisole 2,6-Dimethylanisole 2-Ethylanisole 2,5-Dimethylanisole 2-iso-Prop ylanisole
2,4-Dimethylanisole 3,5-Dimethylanisole 2-n-Propylanisole 2,3-Dimethylanisole 2,4,6-Trimethylanisole 2-tert-Butylanisole 3,4-Dimethylanisole 2-Methyl-4-ethylanisole 2,3,6-Trimethylanisole 3-Methyl-5-ethylanisole 2,4,5-Trimethylanisole 4-tert-Butylanisole 2,3,5-Trimethylanisole 2,3,4-Trimethylanisole 2,6-Di-tert-butylanisole
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Boiling pt "C 111.0 154.0 171.O 176.0 177.0 182.0 187.0 194.0 200.0 192.0 193.0 208.0 199.0 202.0 ... 204.0
...
... ...
214.0 222.01 730 mm 216.0 ...
...
ANALYTICAL CHEMISTRY
Relative retention time 1.00 1.50 2.00 2.25 2.34 2.74 2.88 3.38 3.37 3.50 3.74 3.75 3.81 4.30 4.37 4.37 4.88 4.94
...
1 .oo
1.33 1.50 1.56 1.83 1.92 2.25 2.25 2.33
5.41 6.00
2.49 2.50 2.54 2.87 2.92 2.92 3.25 3.29 3.33 3.61 4.00
6.53 7.75 20.16
4.35 5.16 13.45
5.00
The reaction is exothermic and for quantitative work the temperature should be well controlled so that no mist escapes, When the reaction has subsided and no mist can be seen in the flask, the mixture is allowed to reflux for 2 hours, After this, it is cooled and excess sodium is decomposed by adding slowly and carefully ice cold methanol. Excess DMS is decomposed by refluxing the mixture (1 hr) after adding sufficient sodium hydroxide solution (20%). The oily layer of the aryl methyl ether is separated, the aqueous layer is extracted twice with small quantities of diethyl ether, the ether extract along with the oily layer is washed twice with distilled water and then dried over fused calcium chloride. The dried ethereal solution is then distilled to remove most of the ether. This operation must be carried out very carefully so that no anisoles escape. Best results have been obtained by immersing the distillation flask on a water bath, the temperature of which is not allowed to exceed 55 "C. and the distillate temperature is kept below 36 "C. The concentrated ethereal solution of the anisole is then cooled and 0.2 p1 from this is directly injected into the chromatograph with the help of a 10-pl Hamilton syringe. Distillation of Tar Acids. Two samples of tar acids obtained from hydrogenation experiments of Assam coal (details to be published elsewhere) and one sample of tar acids (up to 250 "C) obtained from a commercial high temperature carbonization pilot plant of this Institute were first fractionated under reduced pressure (40 mm of Hg) in a spinning band distillation column. The hydrogenation tar acids were distilled up to 235 "C (calculated for 1 atm). The distillation of H.T. tar acids was continued till the still was almost dry and distillate boiling up to 210 "C (calculated for 1 atm) was collected. The distillates were then subjected to methylation by the above procedure. DISCUSSION
Retardation of electrophillic attack on alkyl phenols may be ascribed to two different factors; first, the weakening of acidity due to the inductive effect of the electron-repelling alkyl groups and second, to the steric hindrance to solvation in case of ortho substituted phenols. Thus, in one preliminary experiment, a solution of 4,6,di-terf-butyl-2-methylphenolin Dioxan when only warmed on water-bath with molecular sodium followed by subsequent treatment with DMS led to an incomplete formation of the corresponding anisole. However, when this phenol dissolved in Dioxan was refluxed for 2
Table 11. Relative Retention Times of Methyl Ethers of Some High Boiling Phenols (Apiezon L at 180 "C) Boiling Relative retention time Compound pt "C Toluene 3,4,5-TrimethyIanisole 4-Methoxyindane 5-Methoxyindane
111.0
5-Methyl-4-methoxyindane 4,6-Di-tert-butyl-2-methylanisole
226.0 234.0
... ...
10.50 10.86
1.00 1.35 1.45 1.60 2.10 2.17
12.25 13.95 14.10 14.20 15.00
2.45 2.79 2.82 2.84 3.00
...
6-Methyl-Smethoxyindane 5,6,7,8-Tetrahydro-2-methoxynaphthalene ... 1-Methoxynaphthalene 268.0 ... 2-Cyclohexylanisole 2-Methoxynaphthalene 273.0 2-Phenylanisole 274.0
I
I
I
I
IO
8
6
4 MlNS
I
I
2
0
-
Figure 3. Gas chromatogram of a synthetic mixture of high boiling phenols after 0-methylation hours in presence of molecular sodium before treatment with DMS, the reaction was virtually complete. Refluxing in Dioxan in presence of molecular sodium was found to be esential for complete etherification with DMS. Under such conditions, even the most refractory compound like 2,6-ditert-butylphenol, where the two bulky hydrocarbon groups shielding the location of a n electrical charge would seriously interfere with solvation, is methylated t o the extent of about 98%.
Thus 20% of the phenol was found to remain unreacted (Figure 1 a) when 2,4,6-trimethylphenol was sought to be methylated by Bergmann's procedure whereas the methylation was quantitative (Figure 1 6 ) when carried out by the method described in the paper. In all, 34 phenols were quantitatively methylated by this procedure. Choice of phenols studied in this work was governed only by their availability and in some cases reactions had to be carried out with only 30-mg quantities. In Table I, retention times of methyl ethers of low boiling phenols relative to that of anisole on Apiezon L at 140 "C and in Table I1 those of some high boiling phenols relative to 3,4,5trimethylanisole at 180 "C on Apiezon L are given. Since some toluene was invariably present in all our reaction products, retention times with respect to toluene are also given therein. Two synthetic mixtures of phenols were analyzed via methylation. Isothermal chromatogram at 140 "C of one of the mixtures after methylation is shown in Figure 2 and the results are given in Table 111. Results (not shown) obtained with the programmed temperature chromatography of the second sample were equally accurate. Finally in Tables IV and V are given the results of analysis of two coal hydrogenation tar acids and one HTC-tar acid as obtained by following the methylation procedure as described in this work. Assignments have been based solely on the relative retentions given in Table I. Apiezon L is not an ideal stationary phase for the analysis of phenols or their methyl ethers. Complete resolutions up to xylenols could be achieved on packed columns with phases
...
1.00 5.00 6.75 7.25
...
8.00
Table 111. Analysis of a Synthetic Mixture of Phenols (Apiezon L at 140 "C) Per cent comoosition w/w (as ph'enol) Peak Component Found Present no. 1 3-Methylanisole 13.39 13.87 2 2,6-Dimethylanisole 7.38 6.53 3 2,5-Dimethylanisole 11.80 11.84 3,5-Dimethylanisole 4 23.60 23.70 5 2,4,6-Trimethylanisole 9.58 8.62 6 2-Methyl-4-ethylanisole 8.36 9.63 7 4-tert-Butylanisole and 2,3,5-TrimethyIanisole 22.35 22.07 8 2,3,4-Trimethylanisole 3.54 3.74 Table IV. Analysis of Tar Acids from Hydrogenation of Assam Coals Sample l,a Sample 2,* %, of of Compound distillate distillate Phenol 3.27 1.47 2-Methylphenol 8.23 5.99 3-, and 4-Methylphenols 15.02 8.76 2,6-Xylenol and 2-Ethylphenol 8.35 7.52 2,4-, 2,5-, 3,5-Xylenols; 2-iso-Propylphenol, and 37.30 31.58 2-n-Propylphenol 2,4,6-Trimethylphenol and 3,4-xylenol 13.15 14.70 2,3,6-Trimethylphenol, 2-Methyl-Cethylphenol and 10.20 18.30 3-Methyl-5-ethylphenol 2,4,5-Trimethylpheno1and 4.48 11.68 1 2,3,5-Trimethylphenol a Yield of distillate, 50 on total. Yield of distillate, 46.4% on total.
z
1
1
z
Table V. Analysis of a Tar Acid from High Temperature Carbonization of Coal Compound of Distillate Phenol 22.30 2-Methylphenol 22.10 3-, and CMethylphenols 41 .OO 2,6-Xylenol 3.90 2,4-, and 2,5-Xylenols 10.70 Yield of distillate, 70.5 on total.
z
z
VOL. 40, NO. 12, OCTOBER 1968
0
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like tri-2-4-xylenylphosphate (8) and di-(3-3-5 trimethylcyclohexyl)-0-phthalate (9) or even better, a mixed phase of these two compounds (IO). Finally, Figure 3 shows the chromatogram obtained after methylation of a synthetic mixture of high boiling phenols. Due to non-availability of pure phenols in this boiling range, quantitative results could not be given. However, the poten(8) V. T. Brooks, Chem. Ind. (London), 1959, 1317. (9) W. Sassenberg and K. Wrabetz, 2. Anal. Chem., 184, 423 (1961). (10) C. Landault and G. Guiochon, ANAL.CHEM., 39, 713 (1967).
tial usefulness of the methylation procedure described in this paper for quantitative gas chromatography of high boiling phenols is obvious from the shape of the chromatographic peaks. ACKNOWLEDGMENT
The authors are grateful to C. S. B. Nair for making them a gift of his valuable stock of phenols without which the present work could not be undertaken. Thanks are due to A. Lahiri, Director, for his kind permission to publish this paper. RECEIVED for review March 26, 1968. Accepted June 24, 1968.
Portable Atomic Absorption Photometer for Determining Nanogram Quantities of Mercury in the Pres,enceof Interfering Substances Clement Ling Kodak (Australasia) Pty. Ltd., Research Laboratories, P.O. Box 90, Coburg, 3058, Victoria, Australia THE POTENTIAL SENSITIVITY of atomic absorption is not realized in the conventional flame method because only a very small amount of the atomized sample occupies the optic ml at any path. (The amount is estimated to be only given instant.) The limitation in sensitivity is further imposed by the dynamic requirement of the sample solution to be sprayed continuously into the flame. In a conventional flame, an analyte with a concentration of 1 pg/ml (1 ppm) will have only lod5 a pg in the optic path. If however, a static is used-i.e., system in which a given sample may be atomized completely in a cell-the concentration of an analyte required to produce pg would be inversely proportional to the volume of sample taken. The analytical range may be further extended by the application of extraction technique. Atomization in a static system presents a problem because the atomic vapor is unstable. L’vov ( I ) , however, has developed a static system in which sample deposited on a secondary electrode after having been evaporated is atomized by arcing with a primary electrode consisting of a heated graphite cell into which atomic vapor is introduced. There are some elements which are easily atomized by moderate heat and their vapors under normal conditions are nonatomic. Such elements lend themselves readily to atomic absorption by static system. Mercury is an outstanding example. The optimum conditions for atomic absorption have been discussed previously (2) and a method of correcting for nonatomic absorption has also been proposed (2). EXPERIMENTAL
In a preliminary experimental arrangement (2), the resonance lamps (argon filled and highly evacuated) are individually excited by a G.E. (OZ4Sll) ozone producing lamp. The resonance lamp emitting a broadened line is passed through a cell containing a drop of mercury and filled with nitrogen to simulate the absorption line profile in the absorption cell. (It was found later that this desensitizing cell must be heated (1) B. V. L’vov, Spectrochim. Acta, 17, 761 (1961). (2) C. Ling, ANAL.CHEM., 39,798 (1967).
1876
ANALYTICAL CHEMISTRY
to 70 “C before the broadened line may be rendered insensitive to mercury vapor. To prevent photosensitized oxidation of the mercury and its vapor, about 100 mm of hydrogen was also added to the desensitizing cell.) The signal from each resonance lamp is transmitted through the same absorption cell individually to the detector (1P28) by means of a shutter. As it emits a stronger signal, an attenuator (comb type) is placed between the sharp resonance lamp and its exciting lamp. The signal from the broadened resonance lamp is first adjusted to 100% transmittance by adjusting the gain (EHT to the photomultiplier), and the output is connected directly to a recorder. The signal from the sharp resonance lamp is then adjusted to 100% transmittance by means of the attenuator. The vapor may be introduced into the cell by suction through the side arm or by placing the sample in the sample test tube. When mercury vapor is introduced so that 99% signal of the sharp resonance line is absorbed, the broadened resonance line shows only 2% absorption, whereas €or organic vapors such as acetone, benzene derivatives (such as xylene and chlorobenzene), and cigarette smoke, both signals are absorbed to the same extent. This means that if nonatomic absorption is present with mercury vapor, the 100% level from the two signals will be reduced and the signal from the sharp resonance lamp will show greater absorption. The true atomic absorption is now the ratio of the signal from the sharp resonance lamp to the signal from the broadened resonance lamp. The sharp resonance lamp will read atomic absorption in % T simply by adjusting the gain (EHT to the photomultiplier) until the signal from the broadened resonance lamp reads 100% T. The optical system is subsequently simplified by using only one exciting lamp whose radiation is collimated and reflected onto the appropriate resonance lamps, Figure 1. Two conventional ways of introducing the vapor into the absorption cell for measurement are the difusion method (3): Sample test tube directly attached to the side arm of the absorption cell and the vapor generated then allowed to diffuse into the cell; and the continuous flow method ( 4 ) : The sample (3) A. E. Ballard and C. D. W. Thornton, IND. ENG.CHEM., ANAL, ED., 13,893 (1941). (4) E. G . Pappas and L. A. Rosenberg, J. Assoc. Ofic.Anal. Chemists., 49, 782 (1966).
